Ulster star Best is expected to return home within the next 24 hours after medical results last night revealed his condition was related to a cardiac issue rather than a neurological problem.

A statement last night from the Ireland camp said that an exact diagnosis of the problem has yet to be determined but revealed that the Ulster star was comfortable and all his symptoms - loss of feeling in his right side, headache and impaired speech - had thankfully been resolved.

Best's wife Katie flew out to Bordeaux to be at her husband's side yesterday.

And Ireland coach O'Sullivan and forwards coach Niall O'Donnovan visited the 29-year-old last night to join Ireland team doctor Gary O'Driscoll, who has been at Best's side since he was first admitted to the Pellegrin hospital in Bordeaux on Wednesday night after suffering the symptoms while on a trip to the city centre with Ulster colleague Paddy Wallace.

An IRFU spokesman said today they had found him in "great spirits" . Best's father John will arrive in the French city today.

O'Sullivan also last night made his call on the vacant hooker's berth, with Simon's brother Rory named on the bench having recovered from a thumb injury.

Rory had been first choice before he picked up the injury in the victory over Georgia and was expected to return to the side in place of Jerry Flannery, who now keeps his place.

"We have named Rory Best on the bench for this weekend and while we did consider his brother Simon's condition when making that decision, Rory is an incredibly mentally tough individual and was adamant that he wanted to be considered for selection," added O'Sullivan. "We have no doubts that he will be ready if called upon."

Simon Best's position on the bench is filled by Ulster prop Bryan Young while Munster's Tony Buckley has been called in to the World Cup squad.

As the team travelled to Paris today leaving Best behind, O'Sullivan now wants his side to put the last harrowing 24 hours behind them and focus on keeping their World Cup bid alive.

Ireland must defeat Argentina and score four tries while preventing their opponents getting a losing bonus point to reach the quarter-finals.

O'Sullivan said: "We have an issue that we didn't expect to have with one of our friends, one of our family

"We have to get on with our preparation because the game is going to go ahead regardless. It's just about getting that balance right in our heads. We've got to deal with the fact we've got a game on Sunday."